Pain enhances naloxone-induced hyperalgesia in humans as assessed by somatosensory evoked potentials

The effect of 8 mg IV naloxone on pain appreciation was studied with electric shocks administered to the left forearm of 20 normal volunteers. Pain sensitivity was assessed with a psychophysical task and with evoked potentials (EP) to the pain stimuli which were found sensitive to opiate agonists and antagonists in previous experiments. Naloxone-induced hyperalgesia before and after 20 min of intermittent shock was assessed in a 3-day placebo crossover experiment designed to provide control comparisons of time effects. EP amplitude enhancement with naloxone was significantly greater following 20 min of shocks than preceding them, while pain judgments were not significantly affected. Thus, naloxone increases pain sensitivity, especially after prolonged pain stimulation. This finding is consistent with endorphin mediation of stress-induced analgesia and raises the question of whether this type of response decrement over time is related to the phenomena of habituation.